1. Field of the Invention
This invention relates to a thermal flow sensor for vehicles, and to a power source thereof for driving a bridge circuit unit and an integrated circuit (IC) unit for controlling the bridge circuit unit and outputting a signal of a detected flow rate of a fluid.
2. Description of the Related Art
In a thermal flow sensor for vehicles, a heating resistor is heated so that the temperature of the heating resistor becomes a predetermined temperature higher than the temperature of a fluid detected by a resistance thermometer for detecting the fluid temperature. The thermal flow sensor is heated by passing a current through the heating resistor so that the temperature of the heating resistor becomes a predetermined temperature higher at all times than the fluid temperature detected by the resistance thermometer. Since the heat quantity transmitted from the surface of the heating resistor surface to the fluid varies according to the flow rate of the fluid flowing on the surface of the heating resistor, the current supplied to the heating resistor is dependent on the flow rate. The flow rate of the fluid can be detected by converting this current into voltage, adjusting it to fall within a desired output characteristic tolerance, and outputting the adjusted voltage. A specific example of this type of the thermal flow sensor is shown in Patent Document 1. An example of another type of the thermal flow sensor is shown in Patent Document 2.
[Patent Document 1] JP-A-2000-314645
[Patent Document 2] JP-A-2005-283381
In the case of automobiles, conventional techniques widely use a system in which the power for the thermal flow sensor is supplied from a battery via a power source terminal.
The supplied power (electric power) is supplied separately to a bridge circuit unit and an integrated circuit unit in the thermal flow sensor. It is necessary to protect the integrated circuit unit so as not to break because of battery line over voltage or superimposed surge. For this purpose, an approach of providing a protection circuit or a regulator in a previous stage of the integrated circuit unit and an approach of increasing the withstanding voltage of the integrated circuit unit have been widely used, which prevent cost reduction and size reduction of the thermal flow sensor.
In the conventional technique shown in Patent Document 2, a constant voltage source (ECU power source) stabilized by a fuel injection control device, in the case of automobiles, may be used as the power source for the thermal flow sensor. However, because the constant voltage stabilized by the fuel injection control device is about 5 V, it is necessary to set the temperature coefficient of resistance of the heating resistor to 1000 ppm/° C. to 2000 ppm/° C., in order to ensure the electric power necessary for heating the heating resistor to a predetermined temperature. This imposes the constraint that the material for the heating resistor is limited.
It is an object of this invention to provide a thermal flow sensor for vehicles that protects the integrated circuit (IC) unit from battery line overvoltage and superimposed surge, without being restricted by the heating resistor with a constrained temperature coefficient of resistance.